Rats likely decimated Ewa forest

Oahu's Ewa Plain once had a large native dryland forest that suddenly disappeared, similar to Easter Island

Oahu's Ewa Plain once had a large native dryland forest that suddenly disappeared, similar to the forest on Easter Island, according to archaeological studies.

Evidence from a large sinkhole known as Ordy Pond indicates the main reason for the Ewa forest's decline was the Polynesian rat, brought here by early colonists, said J. Stephen Athens of International Archaeological Research Institute Inc.

Athens, who specializes in paleo-environmental research, and colleagues took a sediment core from Ordy Point in the late 1990s. They studied the pollen record to evaluate change and the possible impact humans had on the environment in the past, Athens said.

The record went back about 1,500 years, based on radiocarbon dating of the sediments, he said in discussing the project in an interview.

Analysis of the pollen showed "a very abrupt transition approximately 1,000 years ago," when the dryland forest rapidly disappeared -- "just poof," he said.

Palm trees such as Pritardia, which had dominated the forest, were replaced mostly by grass and weeds, he said.

While studying the Ordy Pond results, the researchers also undertook a large survey and pulled together all archaeological work done on the Ewa Plain, Athens said.

They excavated a number of small sinkholes that didn't have ponded water but contained a lot of fossil bird bones that were examined by Smithsonian avian specialists. They documented a large collection of bird species never known to exist, he said.

"The coring work in the pond kind of helped us put these different lines of information together."

Part of the Ewa Plain project was to obtain accurate radiocarbon dates on bones of Pacific rats introduced by Polynesians, Athens said.

He said they couldn't pinpoint when the rat first appeared, but six to eight specimens dated tend to be early in terms of Hawaiian settlement. It's believed the rat population exploded without predators or competition, radiated out across the island and destroyed native vegetation that many native bird species depended on, he said.

Archaeological data indicate that initial human settlement may have taken place as early as 1200, but the forest seemed to decline well before any evidence of humans in the region, Athens said.

It had been thought that early Polynesians were responsible for burning or clearing the forest, he said. "But here for the first time we could see there were really no humans out on the Ewa Plain."

Microscopic charcoal particles in the core indicate some human activity around 1000, but no fires before that, he said.

Extinct bird bones are rarely found in archaeological middens, and there is no evidence people were exploiting the resource to any extent, he said. "They were probably already gone, the extinct duck, goose and other things.

"So the rat, at least in theory, seems like a very good candidate for having the potential to cause a great deal of destruction."